This invention pertains to power electronics design and manufacture.
IMS circuit board materials are comprised of a metal substrate, usually aluminum or copper with a typical thickness from 0.040″ to 0.125″. A thin insulating material is bonded to the substrate and a layer of copper foil is bonded to the insulating material.
The IMS material is processed into printed circuit boards in much the same way as a typical fiberglass PCB where a photo mask is applied to the copper foil and the unwanted copper is chemically etched away, leaving the desired traces and pads.
For high power applications, IMS printed circuit boards have only one usable layer and are only suitable for surface mount components. Fiberglass boards can have many layers and a mix of through-hole and surface mount components. When power semiconductor devices are soldered onto an IMS printed circuit board, very low thermal resistances from the power components to the metal substrate are had. In high power applications, the metal substrate is in turn mounted to a heatsink. Low thermal resistance enables higher power to be processed with less silicon die area, which translates to lower costs and higher volumetric power density. The soldered connection of power devices to the heat transfer surface is much more reliable than any mechanical clamping method. Also, robotic pick-and-place assembly methods can be used on IMS PCB's assemblies having large numbers of surface mount power devices.
There are, however, two problems with the IMS materials for high power applications. First, the mechanical strength of the bond between the copper and insulating material and insulating material and substrate is limited. This weakness precludes the use of physically large, soldered, surface mounted terminals to hold large cables. A typical prior art method uses multiple, low-current, surface mount connectors, pins or headers to make the transition to a fiberglass PCB where a single, high current, high mechanical strength termination can be made. Second, the metal substrate layer is typically used only for mechanical support and heat transfer. It is desirable in most switch mode power converter applications to have a low AC impedance DC bus which requires a two layer circuit board or other laminated bus assembly. The IMS material is limited by effectively having only one layer and moreover cannot except the larger through-hole electrolytic capacitors typically required to achieve a low AC impedance DC bus.
Another prior art method disclosed under U.S. Pat. No. 5,715,141 by Karlsson describes a method of using conductive metal tubes between an IMS PCB and a fiberglass PCB. Machine screws are inserted through clearance holes in a top fiberglass PCB, through the center of the metal tubes, through clearance holes in a bottom IMS PCB and are threaded into a heatsink. The method provides local clamping pressure, at the screw and tube locations, between the IMS PCB and the heatsink. The tubes also provide current paths between the fiberglass PCB and the IMS PCB and act as spacers between the two PCBs.
The invention disclosed by Karlsson is limited to conductive spacing and clamping elements having a specific tubular geometry. It is not clear from Karlsson's patent that the method of conducting current from one PCB to another PCB using a tubular spacer or standoff is claimed as novel, a method that was obvious and known prior to the Karlsson patent.
The new invention uses a novel geometry and assembly for conductive spacing and clamping elements which allow the clamping pressure between an IMS PCB and a heatsink to be distributed over a much greater surface area compared to the prior art. The new invention also enables very low current densities to be achieved at board-to-board connections and in board-to-board connectors compared to the prior art. Both characteristics are a direct result of the bus bar geometry disclosed in the new patent and a novel method of easily facilitating the incorporation of these bus bars.
Other prior art methods, including the Karlsson patent, do not provide a method of collecting or summing the currents in multiple conductors (tubes in the Karlsson case) into a higher current conductor. The prior art does not include a method of making high current connections to external power sources and loads to/from IMS substrates. Most importantly, prior art methods do not enable low AC impedance connections to be made from the semiconductor switching devices on the IMS PCB to filter capacitors or other components on a second PCB, a key requirement in high frequency power converters.
What is new and novel is an arrangement of a heatsink, an IMS printed circuit board with power semiconductors, bus bars of a specific geometry, a fiberglass printed circuit board with capacitors and a mechanical clamping method which together allow surface mount power semiconductor switching devices to optimize the superlative heat transfer properties of IMS materials and operate from a very low AC impedance DC bus, characteristics that were mutually exclusive in the prior art.
With this invention, the bus bars perform multiple functions by spreading the clamp force to provide low resistance electrical contacts with the PCBs, distributing the clamp force between the IMS board and the heatsink for enhanced heat transfer, acting as a spacer between PCBs, providing a means to interface external sources and loads with the IMS PCB, presenting low DC impedances to currents in the bus bars and providing a low AC impedance interface to circuit boards mounted to the bus bars.
What is old and known are the power circuit topologies used on the IMS printed circuit board and the control methods commonly used with these circuit topologies to perform power conversion.
What is old and known is how to form a low AC impedance DC bus on a fiberglass printed circuit board by using different layers as power planes for various DC bus poles.
What is old and known is the composition and characteristics of Insulated Metal Substrates (IMS) materials.
FIG. 1 shows a section view of the invention preferred embodiment.
FIG. 2 shows the top view of the IMS printed circuit board referenced in FIG. 1.
FIG. 3 shows the top view of the fiberglass printed circuit board referenced in FIG. 1.
FIG. 4 shows the electrical schematic for the preferred embodiment of the invention illustrated from an electromechanical perspective in FIG. 1.
FIG. 5 shows a simplified schematic of the half-bridge circuit topology used for the preferred embodiment illustrated in FIGS. 1 and 4.
FIG. 6 shows a simplified schematic for a half-bridge circuit topology that incorporates a bipolar DC bus.
FIG. 7 shows a simplified schematic for a full-bridge circuit topology intended for use with a mono-polar DC source.
FIG. 8 shows a simplified schematic for a full-bridge circuit topology intended for use with a bipolar DC source.
FIG. 9 shows a simplified schematic for a three-phase bridge circuit topology intended for use with a mono-polar DC source.
FIG. 10 shows a simplified schematic for a three-phase bridge circuit topology intended for use with a bipolar DC source.